1. Field of the Invention
The present invention relates to a method and system for separation of gas mixtures into gas volumes having higher concentrations of selected constituent gases, and particularly to such methods and systems as operate at relatively low pressures, such as partial atmospheric pressure, or less.
2. Description of Related Art
It is generally desirable, for a variety of applications, to separate constituent gases of a gaseous mixture. For example, the constituent gases of oxygen, nitrogen, carbon dioxide, argon, etc., are often separated from air, using numerous different methods. Methods for such separation often entail the use of massive and/or complex equipment, and consume a large amount of energy. Consequently, bottled gas (pressurized or liquefied) is predominantly used for applications in which a relatively pure gas is required. Except for a few specialized industrial processes, continuous, on-demand gas separation methods that use ambient air as a feedstock are generally too expensive and/or not technically feasible for many applications in which gaseous separation may be beneficial.
For example, for combustion engine applications, it may be beneficial to enrich intake air with oxygen, to enable increased power or efficiency. Presently, there is no effective solution for providing oxygen in a continuous process using air as a feedstock, that is feasible for use with gasoline or diesel engines in transportation applications. Similar benefits might be realized with other mobile or stationary power plants or combustion applications. Separation of more concentrated oxygen and/or nitrogen streams from ambient air may also be useful for various different industrial processes, for fire prevention or suppression, for air conditioning or medical purposes, and/or other applications.
It is desirable, therefore, to provide a system and method for separating, at least in part, constituents of a gaseous mixture. The system and method should be capable of separating the primary constituents of ambient air (i.e., oxygen and nitrogen) using relatively light-weight equipment that is economical and relatively maintenance-free. The equipment should be useful for diverse applications, for example, for use with diesel or gasoline engines in trucking, marine, or automotive applications, or for any other application where a continuous gas stream that is at least partially enriched in oxygen or nitrogen is desired.
The invention provides a method and system for separating constituents of gas mixtures, that is suitable for separating oxygen, nitrogen, and perhaps other constituents, from ambient air in a continuous process. The method may be implemented using lightweight, relatively inexpensive equipment that may be configured for a variety of different applications and operating environments. The invention is therefore believed suitable for use with variety of different applications for which continuous, on-demand gas separation was previously not feasible.
According to a method of the invention, a gas mixture is continuously introduced into an ionization chamber. The ionization chamber comprises oppositely-charged electrodes separated by an enclosed volume that is filled with the gas mixture. Each of the oppositely charged electrodes faces the enclosed volume. The electrodes may be generally planar or sheet-like, with a primary surface of relatively large area. In the alternative, the electrodes may be configured as wire or needle arrays. Each electrode has a second surface facing an exhaust plenum, and generally divides the enclosed volume between the electrodes from a separate exhaust plenum. By arrangement of the electrodes, the ionization chamber comprises at least one enclosed volume bounded by at least two oppositely-charged electrodes, and at least two exhaust plenums, each bounded by a single electrode. Each electrode further comprises at least one passageway connecting the enclosed volume between the electrodes to the exhaust plenum on the opposite side of the electrode, to permit gas to flow through or around the electrode. The ionization chamber, including the volume between the electrodes and the two exhaust plenums, may also be bounded by non-electrode surfaces that may be either non-conductive or conductive. If other boundary surfaces are conductive, they are insulated from the electrodes.
Each exhaust plenum is connected to an exhaust port through which an exhaust gas stream is drawn at a controlled rate. The gas mixture in the volume between the electrodes is maintained at a controlled pressure that is generally less than atmospheric pressure, for example, a low vacuum pressure. In an embodiment of the invention, gas pressure is maintained by adjusting a gas input valve connected to the volume between the electrodes, depending on the exhaust rate. By throttling the input valve while pumping gas out the exhaust port, a continuous flow of low-pressure gas can be drawn through the volume between the electrodes and out the separate exhaust ports.
The electrodes are connected to opposite terminals of a DC voltage source, thereby establishing a static electric field between the plates. Electrode voltage should be selected to promote ionization of the gas mixture in the input space, while avoiding generation of any unwanted ion species. Optimum voltage will depend on parameters such as the chemistry of the gas mixture; spacing, shape, and composition of the electrodes, and gas flow rate. In general, higher gas pressures may facilitate higher mass flow rates, while requiring higher electrode voltage. If gas pressure is too high, however, separation of different ionic species may be impaired.
The electric field between the electrodes may cause a portion of the gas in the input space to become ionized. In an embodiment of the invention, the rate of ionization is increased by exposing the gas in the input space to ionizing radiation, such as from an ultraviolet lamp or other radiation source. In another embodiment, the electrodes may by themselves provide adequate ionization, without a further radiation source.
Separation of the gas species from the mixture proceeds as the gaseous mixture between the electrodes is ionized. In many gas mixtures, different gas species of the mixture will possess a greater affinity for electrons than other species of the mixture. Hence, when the gas mixture is ionized, the negative ions will be made up of a proportionally greater number of the gas species having a higher electron affinity, depending on factors such as the electric field strength, the type of gas, and the density of the gas. For example, in a mixture of oxygen and nitrogen, oxygen has a greater affinity for electrons, so under certain conditions, a greater proportion of the negative ions will be oxygen, relative to the proportion of oxygen in air. Conversely, more of the positive ions will be nitrogen. Creation of negative O2 ions and positive N2 ions may predominate when the electric field is less than required to produce an arc discharge. For example, at atmospheric pressure, less than about 20,000 Volts per cm.
The oppositely charged electrodes define opposite surfaces of the input space, and so the negative ions may propagate towards the positive electrode, and the positive ions may propagate towards the negative electrode. As the ions propagate towards their respective electrodes, they are drawn through the electrode passageways and into the exhaust space by suction applied through the exhaust port. In the process, the charged ions may be essentially neutralized by the oppositely charged electrode through which they are being drawn. The gas in the exhaust space, now enriched in a desired gas species relative to the mixture, is then drawn out the exhaust port for use in the intended application. For greater enrichment, two or more ionization chambers as described may be placed in series, with the exhaust gas from one chamber fed into the entry port of a second chamber in the series.